In the field of medicine, it is often necessary to know a characteristic property of an anatomical structure. This is in particular a geometric property, for example the position of an axis or plane and/or the position of an axis or plane relative to another axis or plane, wherein the anatomical structure is for example a bone.
The geometric property in particular describes the relative position of “shape representatives” of the anatomical structure. The shape representatives represent a characteristic aspect of the shape of the anatomical structure. Examples of shape representatives are straight lines, planes or geometric figures. Geometric figures can be one-dimensional such as for example axes or circular arcs, two-dimensional such as for example polygons and circles, or three-dimensional such as for example cuboids, cylinders and spheres. Shape representatives can also represent parts of an anatomical structure, for example portions of a bone. The relative position between the shape representatives can be described in reference systems, for example by coordinates or vectors, or can be described by geometric variables such as for example length, angle, area, volume and ratios. The characteristic aspects which are represented by the shape representatives are for example symmetry properties which are represented for example by an axis of symmetry. Another characteristic aspect is for example the direction of extension of the anatomical structure which is for example represented by a longitudinal axis. Another characteristic aspect is for example the cross-sectional shape of an anatomical structure which is for example represented by an ellipse. Another characteristic aspect is for example the surface shape of a part of the structure which is for example represented by a plane or a hemisphere. The characteristic aspect represents in particular an abstraction of the actual shape or an abstraction of a property of the actual shape (such as for example the symmetry property or longitudinal extension).
In the case of a femoral bone, the characteristic property is for example the angle of the femoral neck axis in relation to the axis of the femoral shaft. If the anatomical structure is a spine or part of a spine, then a characteristic property is for example the kyphosis or lordosis angle between two vertebrae. The characteristic property can also be the position of the anatomical structure, for example in relation to another anatomical structure.
Various methods are known for determining a characteristic property from the position of scan data which represents scan points on the surface of the anatomical structure. To this end, a model of the anatomical structure is for example adapted to the scan data, and the property is derived from the adapted model, for example from the position of landmarks of the adapted model relative to each other. Another option is to ascertain an axis and/or plane, as an equalization axis and/or equalization plane, from the scan data and to derive the characteristic property from this.